Method of making stress-relieved metal tanks



United States Patent i 3,343,249 METHOD OF MAKING STRESS-RELIEVED METALTANKS Hjalrnar E. Breit, Jr., New Orleans, La., assignor to BreitEngineering Inc., New Orleans, La. No Drawing. Filed July 15, 1966, Ser.No. 565,399 11 Claims. (Cl. 29404) ABSTRACT OF THE DISCLOSURE A methodof forming and stress-relieving in place a metal tank on a transportcarrier, e.g., a barge, for the transportation of liquefied gases suchas ammonia, by joining together metal plates in the place of operation,filling the formed tank with a liquid such as water for approximately anhour and having the filling liquid exert a stress-relieving pressure ofnot less than the designated tank pressure and not more than twice thedesignated tank pressure, the stress-relieving pressure being caused bythe inherent weight of the filling liquid solely or in combination withapplied external pressure. Stress-relief is thereby gotten withoutresort to the relatively complex and expensive prior art methods ofstress-relief, e.g., annealing the tank in an oven at high temperatures.

This patent application is a continuation-in-part of the parentapplication for Waterborne Freight-Carrying Vehicles, Ser. No. 396,817,filed Sept. 14, 1964, now Patent No. 3,280,779, granted Oct. 25, 1966.

The invention relates generally to an improved method of making metaltanks, and more specifically, to making stress-relieved metal tanks forthe storage of liquefied gases.

In forming and shaping metal plates into the required shapes and injoining the plates to make a tank, especially a tank that is fabricatedin the place of operation and in close conformance with the shape of acarrier such as the barge described in the parent application, internalstresses are set up in the metal that often results in the failure ofthe fabricated tank under operating conditions unless these stresses arerelieved. Heretofore internal stresses have been relieved by annealingthe completed tank in an oven of sufiicient size at a temperature ofabout 1250 degrees F. Annealing thus precludes fabrication in the placeof operation.

It is an object of the invention to provide-a positive method of makingstress-relieved metal tanks in the place of operation that are free fromstress failure caused by internal-stresses.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims.

Metal plates of suitable tensile strength for the temperatures andpressures to be operationally encountered by the completed tank areformed and shaped into constituent parts of a tank. In the forming andshaping internal stresses are set up in the metal plates. The plates arejoined together to make a liquid-tight tank, and the joining addsfurther to the internal stresses already set up in the metal, regardlessof whether the joining is accomplished by welding or riveting. Thecompleted-inplace tank is then filled with a liquid, preferably water asmost economical, at or above ambient temperatures. The liquid fillingthe tank exerts a pressure therein that is a function of the specificgravity of the liquid and the mass of liquid required to fill the tank.If the capacity of the tank is such that the mass of the filling liquidexerts a pressure greater than the operational pressure but not morethan twice said operational pressure, the tank will be relieved of itsinternal stresses. If the filling liquid exerts a pressure less than theoperational pressure, external pressure may be applied to the fillingliquid in the tank by any of various well known means to raise thefilling liquid pressure within said range of stress relieving pressures,or a filling liquid with a sufliciently higher specific gravity may beused to fill the tank to obtain the same results. If the filling liquidexerts a pressure more than twice the operational pressure, a liquidwith a lower specific gravity to bring the tank pressure within thestated range of stress relieving pressures should be used. For bestresults the tank should be allowed to remain full for a period of aboutone hour.

Although we have described the invention with a certain degree ofparticularity, it is understood that the present disclosure has beenmade only by way of example and that numerous changes in details ofsteps and the arrangement thereof may be resorted to without departingfrom the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. The method of making a stress-relieved metal tank comprising: formingand shaping metal plates as constituent parts for fabricating said tank;joining said plates to make a liquid-proof tank; filling said tank witha liquid mass having a specific gravity for said mass of filling liquidto exert a pressure of not less than the designed tank pressure and notmore than twice said pressure; and maintaining said filling liquid insaid tank for a period of time to relieve stresses in said tank.

2. The method of making a stress-relieved tank as described in claim 1wherein the step of filling said tank with a liquid comprises: usingwater to fill said tank; and applying an amount of pressure on saidfilling water to bring the pressure in said tank within saidstress-relieving range of pressures.

3. The method of making a stress-relieved metal tank as described inclaim 1 wherein said constituent parts for fabricating said metal tankare joined to form said tank in its place of operation.

4. The method of claim 1 in which said period of time is at least onehour.

5. In the fabrication of a metal tank having a designed tank pressure, amethod of forming and stress-relieving said tank comprising the stepsof:

(a) providing pre-formed metal elements to be used as constituent partsof said tank;

(b) by joining said elements together to form said tank, substantialinternal stresses being set up in said tank during the forming of saidtank; and

(c) stress-relieving said tank by filling said tank with a liquid massand having said liquid mass exert a pressure on said tank of not lessthan the designed tank pressure'and not more than twice said designedtank pressure, whereby said tank is stress-relieved.

6. The method of claim 5 in which the stress-relieving pressure is hadby using a suflicient amount of filling liquid mass having a specificgravity which will, without external pressure being applied, exert apressure on said tank of not less than the designed tank pressure andnot more than twice said designated tank pressure.

7. The method of claim 5 in which the proper stress-relieving pressureis obtained by using a filling liquid mass in conjunction with theapplication of external pressure.

8. The method of claim 5 in Which the filling liquid mass is water.

' 9. The method of claim 5 in which the filling liquid mass is allowedto remain in said tank for a period of about one hour.

10. The method of claim 5 in which the forming and stress-relieving ofsaid tank are done in its place of operation.

11. In the manufacture of a transport carrier suitable for carryingliquefied gases and having at least one metal tank, the method of:

(a) forming a metal tank in its place of operation on said transportcarrier by joining together metal plates, internal stresses being set upin said tank during said forming; and

(b) positively stress-relieving said tank in its place of operation byfilling said tank with water and having the water exert a pressure ofnot less than the designed tank pressure and allowing said Water toremain in said tank for at least one hour, whereby said tank isstress-relieved without resort to the relatively complex and expensiveprior art methods of stressrelieving, e.g., annealing in ovens at hightempera- References Cited UNITED STATES PATENTS JOHN F. CAMPBELL,Primary Examiner.

THOMAS H. EAGER, Examiner.

1. THE METHOD OF MAKING A STRESS-RELIEVED METAL TANK COMPRISING: FORMINGAND SHAPING METAL PLATES AS CONSTITUENT PARTS FOR FABRICATING SAID TANK;JOINING SAID PLATED TO MAKE A LIQUID-PROOF TANK; FILLING SAID TANK WITHA LIQUID MASS HAVING A SPECIFIC GRAVITY FOR SAID MASS OF FILLING LIQUIDTO EXERT A PRESSURE OF NOT LESS THAN THE DESIGNED TANK PRESSURE AND NOTMORE THAN TWICE SAID PRESSURE; AND MAINTAINING SAID FILLING LIQUID INSAID TANK FOR A PERIOD OF TIME TO RELIEVE STRESSES IN SAID TANK.